Optical scanning apparatus and image forming apparatus using the same

Abstract

An optical scanning apparatus and an image forming apparatus using the same, overcoming spot rotation due to scanning line curvature and wavefront aberration deterioration, including an incident optical system for guiding beam emitted from a light source to a deflector, and an imaging optical system for forming image of the beam deflected by the deflector on a scanning surface. In sub-scanning section, the beam enters the deflecting surface obliquely to plane perpendicular to a deflector axis. Each of incident and exit surfaces of an imaging optical element is a surface in which a tilt angle of sagittal line changes from on-axis toward off-axis in sub-scanning direction, the tilt angle indicating gradient of normal to sagittal line on meridian line with respect to main scanning section. The incident and exit surfaces each have the same sign for a difference between change rates of axial and off-axial tilt angles of sagittal line.

What is claimed is: 
     
       1. An optical scanning apparatus comprising:
 a light source unit; 
 an incident optical system for guiding a light beam emitted from the light source unit to a deflecting unit; and 
 an imaging optical system for causing the light beam deflected for scanning on a deflecting surface of the deflecting unit to form an image on a surface to be scanned, wherein: 
 the light beam emitted from the light source unit enters the deflecting surface of the deflecting unit in an oblique manner with respect to a plane perpendicular to a rotational axis of the deflecting unit in a sub-scanning section; 
 the imaging optical system consists of a single imaging optical element; 
 each of an incident surface and an exit surface of the single imaging optical element is a surface in which a tilt angle of a sagittal line changes from on axis toward off axis in a sub-scanning direction, the tilt angle of the sagittal line indicating a gradient of a surface normal to the sagittal line on a meridian line with respect to a main scanning section; 
 the incident surface and the exit surface of the single imaging optical element each have the same sign for a difference between an axial tilt angle of the sagittal line indicating a gradient of the surface normal to the sagittal line on the meridian line with respect to the main scanning section and an off-axial tilt angle of the sagittal line indicating a gradient of the surface normal to the sagittal line on the meridian line with respect to the main scanning section; 
 if an incident direction of the light beam entering the deflecting surface of the deflecting unit in an oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is negative, a value of the difference is negative; 
 if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is positive, the value of the difference is positive; and 
 the incident surface and the exit surface of the single imaging optical element each have the same sign for a rate of change of the tilt angle of the sagittal line, which changes from on axis toward off axis in the sub-scanning direction. 
 
     
     
       2. An optical scanning apparatus according to  claim 1 , wherein:
 if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is negative, the off-axial tilt angle of the sagittal line on the incident surface of the single imaging optical element decreases monotonously outside 50%-image height on the incident surface in the main scanning direction, and the off-axial tilt angle of the sagittal line on the exit surface of the single imaging optical element decreases monotonously outside the 50%-image height on the exit surface in the main scanning direction; and 
 if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is positive, the off-axial tilt angle of the sagittal line on the incident surface of the single imaging optical element increases monotonously outside the 50%-image height on the incident surface in the main scanning direction, and the off-axial tilt angle of the sagittal line on the exit surface of the single imaging optical element increases monotonously outside the 50%-image height on the exit surface in the main scanning direction. 
 
     
     
       3. An optical scanning apparatus according to  claim 1 , wherein:
 if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is negative, the rate of change of the off-axial tilt angle of the sagittal line on the incident surface of the single imaging optical element decreases monotonously outside 50%-image height on the incident surface in the main scanning direction, and the rate of change of the off-axial tilt angle of the sagittal line on the exit surface of the single imaging optical element decreases monotonously outside the 50%-image height on the exit surface in the main scanning direction; and 
 if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is positive, the rate of change of the off-axial tilt angle of the sagittal line on the incident surface of the single imaging optical element increases monotonously outside the 50%-image height on the incident surface in the main scanning direction, and the rate of change of the off-axial tilt angle of the sagittal line on the exit surface of the single imaging optical element increases monotonously outside the 50%-image height on the exit surface in the main scanning direction. 
 
     
     
       4. An optical scanning apparatus according to  claim 3 , wherein the incident surface and the exit surface of the single imaging optical element each have an axial sagittal line sectional shape formed as a meniscus shape having a concave surface toward a side of the deflecting unit, and a curvature radius of the sagittal line of the incident surface and a curvature radius of the sagittal line of the exit surface of the single imaging optical element decrease from on axis toward off axis in the main scanning direction. 
     
     
       5. An optical scanning apparatus according to  claim 1 , wherein a curvature radius of the sagittal line of the incident surface and a curvature radius of the sagittal line of the exit surface of the single imaging optical element change from on axis toward off axis in the main scanning direction, and at least one of the incident surface and the exit surface of the single imaging optical element has the curvature radius of the sagittal line which reverses from on axis toward off axis in the main scanning direction. 
     
     
       6. An optical scanning apparatus according to  claim 1 , wherein the imaging optical element consists of a molded lens. 
     
     
       7. An image forming apparatus comprising the optical scanning apparatus according to  claim 1 , and a photosensitive member disposed at the surface to be scanned. 
     
     
       8. An image forming apparatus comprising the optical scanning apparatus according to  claim 1 , and a printer controller for converting code data supplied from an external device into an image signal and for inputting the image signal to the optical scanning apparatus. 
     
     
       9. An optical scanning apparatus comprising:
 a light source unit, 
 an incident optical system for guiding a light beam emitted from the light source unit to a deflecting unit; and 
 an imaging optical system for focusing the light beam deflected for scanning on a deflecting surface of the deflecting unit on a surface to be scanned, 
 
       wherein:
 the light beam emitted from the light source unit enters the deflecting surface of the deflecting unit in an oblique manner with respect to a plane perpendicular to a rotational axis of the deflecting unit in a sub-scanning section; 
 each of an incident surface and an exit surface of the same imaging lens constituting the imaging optical system is a surface in which a tilt angle of a sagittal line changes from on axis toward off axis, the tilt angle of the sagittal line indicating a gradient of a surface normal to the sagittal line on a meridian line with respect to a main scanning section; and 
 the incident surface and the exit surface of the same imaging lens each have the same sign for a difference between a rate of change of an on-axis tilt angle of the sagittal line and a rate of change of an off-axial tilt angle of the sagittal line. 
 
     
     
       10. An optical scanning apparatus according to  claim 9 , wherein the imaging optical system comprises a single imaging lens. 
     
     
       11. An optical scanning apparatus according to  claim 9 , wherein the imaging optical system comprises two imaging lenses, and the incident surface and the exit surface of the same imaging lens on a side of the surface to be scanned are each formed as the surface in which the tilt angle of the sagittal line changes. 
     
     
       12. An optical scanning apparatus according to  claim 11 , wherein in each of the incident surface and the exit surface of the same imaging lens on the side of the surface to be scanned, a sign for a first difference between the rate of change of the tilt angle of the sagittal line on axis and a rate of change of the tilt angle in one of two off-axial sides of the sagittal line and a sign for a second difference between the rate of change of the tilt angle of the sagittal line on axis and a rate of change of the tilt angle in the other of the two off-axial sides of the sagittal line. 
     
     
       13. An optical scanning apparatus according to  claim 9 , wherein the same imaging lens consists of a molded lens. 
     
     
       14. An image forming apparatus comprising the optical scanning apparatus according to  claim 9 , and a photosensitive member disposed at the surface to be scanned. 
     
     
       15. An image forming apparatus comprising the optical scanning apparatus according to  claim 9 , and a printer controller for converting code data supplied from an external device into an image signal and for inputting the image signal to the optical scanning apparatus. 
     
     
       16. An optical scanning apparatus comprising:
 a light source unit; 
 an incident optical system for guiding a light beam emitted from the light source unit to a deflecting unit; and 
 an imaging optical system for causing the light beam deflected for scanning on a deflecting surface of the deflecting unit to form an image on a surface to be scanned, wherein: 
 the light beam emitted from the light source unit enters the deflecting surface of the deflecting unit in an oblique manner with respect to a plane perpendicular to a rotational axis of the deflecting unit in a sub-scanning section; 
 each of an incident surface and an exit surface of an imaging optical element constituting the imaging optical system is a surface in which a tilt angle of a sagittal line changes from on axis toward off axis in a sub-scanning direction, the tilt angle of the sagittal line indicating a gradient of a surface normal to the sagittal line on a meridian line with respect to a main scanning section; and 
 an absolute value of a difference of a rate of change of the tilt angle of the sagittal line at the same image height between the incident surface of the imaging optical element and the exit surface of the imaging optical element is ⅕ or smaller of an absolute value of a rate of change of the tilt angle of the sagittal line at a most off-axial image height on the respective incident surface and the exit surface of the imaging optical element. 
 
     
     
       17. An optical scanning apparatus according to  claim 16 , wherein the imaging optical system comprises a single imaging optical element. 
     
     
       18. An optical scanning apparatus according to  claim 16 , wherein the imaging optical system comprises two imaging optical elements, and the incident surface and the exit surface of an imaging optical element on a side of the surface to be scanned are each formed as the surface in which the tilt angle of the sagittal line changes. 
     
     
       19. An optical scanning apparatus according to  claim 16 , wherein the imaging optical element consists of a molded lens. 
     
     
       20. An image forming apparatus comprising the optical scanning apparatus according to  claim 16 , and a photosensitive member disposed at the surface to be scanned. 
     
     
       21. An image forming apparatus comprising the optical scanning apparatus according to  claim 16 , and a printer controller for converting code data supplied from an external device into an image signal so that the image signal is received by the optical scanning apparatus.